It's genuinely hard and needs to be improved. Subscribing to a magazine that someone else on kbin has subscribed to already isn't too bad. Go to the magazine (eg, click what looks like the subreddit name in the post) and scroll alllll the way down and there'll be a subscribe button.

But if nobody has subscribed yet in the instance, it's hilariously hard. You have to search in the general search (not the magazine search) for specifically "magazine@domain.com" and you should see a subscribe button then. You will not content in that magazine that existed before you subscribed. If that sounds terrible, it's because it is. Thankfully, most of the time, you won't be the first to subscribe to a magazine and thus can just use the magazine search or browse the front page to see posts.

PS: the subscribe option is also as the bottom of each thread. So you can alternatively just open a thread in the magazine instead of the magazine itself.

PPS: I've mentioned the subscribe button being at the bottom because that's the placement on mobile and I think many of us are on mobile. On desktop, it's in the sidebar.